The present invention relates to an engine unit provided with an oil lubrication system therefor of the type in which a crank shaft is arranged perpendicularly in a state of use, a lubrication oil agitation ring is mounted to the crank shaft of an engine, and when the crank shaft is rotated in a lubrication oil in a crank chamber of the engine, the lubrication oil is circulated in a lubrication circulation passage through a crank chamber wall, a valve moving mechanism, a rocker chamber, etc to the crank chamber.
Small machines such as lawn mower is usually equipped with small engine units each in which a crank shaft is arranged perpendicularly in the state of use and a lubrication system is also arranged for circulating a lubrication oil.
One example of a conventional lubrication system for an engine lubrication oil is disclosed in the Japanese Utility Model Publication No. HEI 6-18008, which is shown in FIGS. 4 and 5. Referring to these figures, a lubrication oil A in a crank chamber K is agitated by an oil splasher S driven by a crank shaft CR of an engine unit to thereby circulate the oil in the engine unit for the lubrication thereof. The oil A is delivered to a rocker chamber R through an upper oil supply passage P to supply the oil to various portions of the rocker chamber R and then return to the crank chamber K through a lower discharge passage Q. These passages P and Q are both opened to the crank chamber K and check valves V1 and V2 are incorporated to these passages P and Q, respectively.
A valve driving mechanism for driving a valve e through a cam a, a tappet b, a push-rod c, and a rocker arm d is disposed to a side portion of a cylinder. The cylinder is formed with a lubrication oil inlet/outlet port L at a level substantially the same height level of the cylinder, and the inlet/outlet port L is also opened to the crank chamber K to lubricate moving parts inside the cylinder.
Furthermore, the Japanese patent Laid-open Publication No. HEI 6-200776 of the same applicant of this application provides a valve moving mechanism for driving a valve, which is not disposed on the side portion of the cylinder and in which a single lubrication oil supply passage to a rocker chamber from a crank chamber is provided. This lubrication oil supply passage also serves as a lubrication oil discharge passage.
As described above, with respect to the oil lubrication systems of conventional engine units, one example shows the structure in which the lubrication oil supply passage from the crank chamber to the rocker chamber and the lubrication oil discharge passage are independently provided, and the other example shows the structure in which the single passage serving as the oil supply passage and oil discharge passage is provided.
In the former example, the check valves are arranged, which makes complicated the structure of the lubrication system, and moreover, in a case where the engine is inclined and the check valve is closed by the lubrication oil, it becomes impossible to guide the lubrication oil flow in one direction, resulting in that the lubrication oil remains excessively in the rocker chamber and the lubrication oil may flow into a combustion chamber, thus being troublesome, and in a case where the crank shaft is inclined, the lubrication oil flows in or out of the cylinder through the lubrication oil inlet/outlet port formed to the cylinder. In such cases, it becomes difficult to suitably maintain the lubricating condition of the lubrication oil.
On the other hand, in the latter example, the lubrication oil supply and discharge are performed through one passage, it is difficult to make balance the supply and discharge of the oil, and in a case where the engine is driven in an inclined state and the opening of the passage to the crank chamber is closed, it becomes impossible to discharge the lubrication oil from the rocker chamber and the oil remains excessively therein and, in an adverse case, the lubrication oil may flows in the combustion chamber through a valve guide, thus providing a problem.